Method and apparatus for turbulent combustion of fly ash

ABSTRACT

An apparatus for processing fly ash comprising a heated refractory-lined vessel having a series of spaced angled rows of swirl-inducing nozzles which cause cyclonic and/or turbulent air flow of the fly ash when introduced in the vessel, thus increasing the residence time of airborne particles. Also disclosed is a method of fly ash beneficiation using the apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 60/691,729, filed Jun. 17, 2005, entitled METHOD AND APPARATUS FORTURBULENT COMBUSTION OF FLY ASH, the disclosure of which is incorporatedby reference in its entirety herein.

FIELD OF THE INVENTION

The present invention relates to methods for processing fine particulatematter, such as coal fly ash, to improve its characteristics, such as byreducing the residual carbon, and removing contaminants, such asmercury, ammonia, and the like. The present invention also relates toapparatus for processing fly ash as well as a fly ash product.

BACKGROUND OF THE INVENTION

Coal fly ash is produced by coal-fired electric and steam generatingplants and other industrial facilities. Typically, coal is pulverizedand blown with air into the boiler's combustion chamber where itimmediately ignites, generating heat and producing a molten mineralresidue. Boiler tubes extract heat from the boiler, cooling the flue gasand causing the molten mineral residue to harden and form ash. Coarseash particles, referred to as bottom ash or slag, fall to the bottom ofthe combustion chamber, while the lighter fine ash particles, termed flyash, remain suspended in the flue gas. Prior to exhausting the flue gas,fly ash is removed by particulate emission control devices, such aselectrostatic precipitators or filter fabric baghouses.

The American Coal Ash Association reports that 70,150,000 tons of flyash was produced in 2003 and that 27,136,524 were beneficially utilized,while the remainder was disposed in lagoons and landfills. The mostprevalent utilization application for fly ash (12,265,169 tons) was inconcrete as pozzolan. Pozzolans are siliceous or siliceous and aluminousmaterials, which in a finely divided form and in the presence of water,react with calcium hydroxide at ordinary temperatures to producecementitious compounds.

A substantial portion of fly ash particles are reactive glass, whichwill combine with alkali hydrates, especially calcium hydroxide, thatare formed as cement hydrates in plastic concrete. This chemicalreaction is referred to as a pozzolanic reaction and the result of thisreaction is a stable cementious bond, similar to the bond that resultsthrough the hydration products of cement, particularly,calcium-silica-hydrate (sometimes referred to as tobermorite gel). Thecementitious bonds produced through the pozzolanic reaction of fly asheffect an increase in the strength and durability of the concrete. Thestrength-producing characteristics of fly ash allow for a lower amountof cement than would otherwise be needed. The value of fly ash aspozzolan is generally related to the cost of the portion of cement thatis replaced by the fly ash.

Due to strategies implemented by electric utilities to meet loweremission limits, whether self-imposed or instituted by governmentregulations, such as the Clean Air Act Amendments of 1990, coal-burningoperations have changed and will continue to change over the nextdecades. Generally speaking, these changes in coal-burning operationsare intended to reduce the emissions of particulate matter or pollutinggases, such as sulfur and nitrogen oxides. Individually these pollutioncontrol methods are designed to limit emissions of, among other things,(1) dust and/or very fine particulate matter, which are associated withincreased rates of hospital admissions, respiratory disease andmortality, especially for mortality due to respiratory andcardiovascular disease for infants and the elderly, (2) fumes of sulfuroxides (SOx), which are directly related to the concentrations andquantities of acid rain, (3) fumes of nitrogen oxides (NOx), which areprecursors for ground level ozone, and (4) mercury and other heavymetals, especially those that are considered to be persistentbioaccumulative toxins.

It is estimated that by the year 2010, these new pollution controlstrategies will result in 6,400 fewer premature deaths, and also createa savings of nearly $40 billion dollars in health care costs. Thesenumbers increase to 12,000 fewer deaths and $93 billion dollars inhealth care savings by the year 2020.

Unfortunately, some of the unintended consequences of these pollutioncontrol methods have negatively impacted the utilization of the coal flyash, especially as pozzolan in concrete. According to the ACAA less than17.5% of the fly ash produced in 2003 was used as pozzolan. However, inmany parts of the U.S., the demand for fly ash as pozzolan issignificantly higher than the local supply of pozzolan-grade fly ash. Amajor reason for this shortage of “quality” fly ash is caused by changesin the characteristics of fly ash produced in the U.S. due to changesmade to coal-burning operations and to the increased use ofpost-combustion pollution control techniques implemented by electricutilities to meet lower emission limits.

According to the U.S. Department of Transportation's Federal HighwayAdministration, changes in boiler operations and/or air emissionscontrol systems at power plants will continue to alter the quality offly ash produced. Factors that impact ash quality in this way include: areduction in the pozzolanic reactivity; the presence in the fly ash ofexcessive unburned carbon; and, chemical residuals from post-combustionemission control.

It would be desirable to beneficially alter the characteristics of coalfly ash, especially the fly ashes that have been negatively affected bythe aforementioned pollution control methods. Generically, suchprocesses are called beneficiation processes; specifically, it would bedesirable to have a thermal beneficiation process that is especiallydesigned to alter these particular characteristics.

There are three major issues which affect the value and utility of flyash used as pozzolan in concrete: the pozzolanic, strength-producingcharacteristics; the air-entraining characteristics; and, the presenceof foreign residual chemicals.

Pozzolanic reactivity is typically measured by the compressive strengthratio between plain portland cement and pozzolan-containing concretes.U.S. ASTM C 618 as presently written specifies minimum “strengthactivity index” performance properties, which compares the compressivestrength of a control concrete specimen made of plain portland cement toa pozzolan-enhanced cement concrete specimen. Unfortunately, sincecomplete strength activity index testing takes at least one month, thereis a substantial lag time before definitive quantification of pozzolanicreactivity can be determined. Fortunately, there is a very goodcorrelation between the specific surface area of the fly ash glass andpozzolanic reactivity with both cement and lime. Therefore, quickcalculation of the specific surface area of the fly ash glass can beused to infer pozzolanic reactivity.

There are several other tests and techniques to infer pozzolanicreactivity. Unfortunately, the correlations between thesesingle-variable test results and pozzolanic reactivity are typicallyvery poor. For example, since pozzolanic reactivity in concrete ismostly related to the reaction between the glass in fly ash and alkalispresent in the cement paste matrix, chemical requirements for the flyash are often used to infer the quantity the reactive glass and,consequently, to classify grades of fly ash and/or predict pozzolanicstrength activity. Specifically, pozzolanic reactivity is mostly relatedto the reaction between the reactive silica glass and calcium hydroxideproducing calcium silicate hydrate. The alumina in the pozzolan willalso react in the cement paste matrix, producing calcium aluminatehydrate, ettringite, gehlenite, and calcium monosulphoaluminate hydrate.ASTM and some other standards associations have also included the ironoxide content as a major requirement.

However, there is a poor correlation between the sum of these oxides(i.e., silica, alumina, and iron) and compressive strength. This may bepartly due to the presence of varying amounts of nonreactive,crystallized phases of silica and alumina (quartz and mullite) and/orinconsistent, and sometimes counterproductive, contribution of ironoxide to strength development. Consequently, using the sum of theseoxides is not considered to be a good technique to infer the strengthactivity of a fly ash.

Therefore, in summary, pozzolanic reactivity can best be quantifiedthrough a “strength activity index,” but determination of specificsurface area for the fly ash glass will approximate changes inpozzolanic reactivity. Increasing the glass to crystalline ratio of flyash will increase the pozzolanic reactivity and, consequently, the valueof the fly ash.

Due to the lower combustion temperatures necessary to reduce theformation of thermal NOx, commercial operation of low-NOx burnersproduce coal ash that has not been exposed to the high-temperatureoperating conditions employed before low-NOx combustion techniques wereimplemented. The operating temperatures employed for low-NOx combustionmay be below the melting point of individual constituents of the mineralmatter contained in the coal being burned, especially for the higher ashfusion bituminous coals. Consequently, the amount of mineral mater thathas become molten and then air-cooled, thereby forming reactive glass,may be reduced significantly due to low-NOx combustion of coal.

There is very little published literature about the reduced pozzolanicreactivity of fly ashes produced through low-NOx burning of pulverizedcoal. However, according to U.S. Department of Energy publications (see,http://www.netl.doe.gov/publications/proceedings/99/99flyash/ellison.pdf),application of low-NOx combustion “ . . . is seen to hinder high-valuefly ash utilization in two ways: (1) much publicized increase inunburned carbon content of fly ash that inhibits commercial use inair-entrained concrete; and, (2) little mentioned, or understood,impairment of ash pozzolanic properties, caused by the greatly reducedfuel firing temperature . . . ”

Decreasing the glass to crystalline ratio of fly ash will decrease thepozzolanic reactivity and consequently the value of fly ash. Thepozzolanic reaction of fly ash glass is also known to increase thelong-term durability of concrete. When fly ash is used as pozzolan inconcrete, the density of the concrete paste matrix may increasesignificantly and, therefore, the durability of the concrete may besignificantly greater than with ordinary portland cement alone.Increasing the glass to crystalline ratio of fly ash will increase thepozzolanic reactivity, making the concrete more impervious and,therefore, more durable.

A second major issue affecting the utilization and value of fly ash—theair-entraining characteristics of the fly ash—is also related to theimpact of fly ash on concrete durability. Particularly, one aspect ofconcrete durability, namely, freeze-thaw durability, may be negativelyimpacted by the presence of fly ash, especially by the presence ofunburned carbon that remains in fly ash following the coal-burningoperation.

There are many factors that can affect the durability of concrete tocycles of freezing and thawing; however, the single greatest impact onfreeze thaw durability derives from the presence and uniformdistribution of air voids in the hardened cement paste matrix withoptimum spacing and size.

When hardened concrete begins to freeze, residual water inside concretewill also freeze; when water freezes its volume increases 9%. Theexpanding ice forces water into the unfrozen regions of the cementbinder. This movement of water creates large hydraulic pressures andgenerates tensile stress. Although concrete has excellent strength incompression, its tensile strength is less than 10% of the compressivestrength. When the tensile stress exceeds the tensile strength of theconcrete, cracking and deterioration occurs. A network of air voids withthe proper spacing and size distribution in the hardened cement pastematrix allows the water to expand and migrate deeper into the concrete,reducing the hydraulic pressure and tensile stress in the concrete.

Air is naturally entrapped in the cement paste of plastic concretethrough the folding and shearing action of the mixing process. However,the entrapped air voids are large and not stable in concrete without theuse of surface active agents, commonly called surfactants. Surfactantscan be used in the production of concrete to reduce the surface tensionof water. Consequently, large air voids will divide into smaller, morestable air voids. Air entraining agents (AEAs) are commonly used assurfactants in the production of concrete designed to increasefreeze-thaw durability.

Residual unburned carbon in fly ash can have a high adsorptive capacityfor AEAs. More specifically, there are certain active sites on thecarbon surface, which are typically nonpolar, that preferentially adsorbAEAs from the aqueous phase. The rate of AEA adsorption will varyaccording to the type, amount, and/or level of activated carbon surfacearea, requiring a varying increase in AEA dosage to maintain the desiredentrained air void system or else resulting in an inconsistent level ofentrained air in the hardened concrete, which will ultimately affect thestrength and/or durability of the concrete by degrading the air voidsystem. There is also an increased risk of over-dosing the AEA andcreating an elevated entrained air content, which would negativelyimpact the strength of the hardened concrete.

The use of fly ash as pozzolan is typically controlled by specificationsthat effectually limit the amount of unburned carbon that can remain infly ash used as pozzolan.

Most specifications prescribe a maximum limit for the Loss On Ignition(LOI) of fly ash used as pozzolan in concrete. LOI is apercent-by-weight measure of the residual combustible material,primarily carbon, in the fly ash. The strength-producing characteristicsof a fly ash are relatively unaffected by LOI levels up to and above12%; therefore, the low maximum limits prescribed by most of thecontrolling specifications for fly ash as pozzolan in the U.S. are notnecessary to assure the strength-producing characteristics. Instead, theintent of these low maximum LOI limits is to assure adequateair-entraining characteristics for pozzolan-grade fly ash used toproduce air-entrained concrete. The concrete industry also referencesspecific LOI values for fly ash to predict and/or monitor theair-entraining characteristics of the various fly ashes available in themarketplace and there is a general perception that lower LOI levelsequate to higher quality.

There are several processes in commercial use that aim to significantlyreduce the LOI of moderate to high LOI fly ashes—to a level below 3% byweight, specifically triboelectric separation and carbon combustion. Itshould be noted that carbon makes up most of the measured LOI (to withinabout 10%); however, as previously discussed, it is the adsorptivecapacity of the fly ash, especially the active carbon sites, for airentraining agents and not the LOI per se, that impacts the marketabilityof the fly ash. At this time, there is a growing realization that lowerLOI fly ashes do not assure superior, or even adequate, air-entrainingcharacteristics for many fly ashes.

Therefore, regardless of the specific reduction of LOI throughcombustion, it would be desirable to beneficially alter theair-entraining characteristics of the processed fly ash by reducing theoverall adsorptive capacity of the fly ash.

A third major issue affecting the utilization and marketability of flyash also derives from operational changes in the coal-burning process,specifically the presence of residual chemicals and/or particulatematter deposited in or adsorbed on the coal fly ash during thecoal-burning operation and/or subsequent flue gas treatment processes,especially those processes intended to reduce air pollution. Thesechanges in coal-burning operations are intended to reduce the emissionsof particulate matter; polluting gases, such as sulfur and nitrogenoxides; and heavy metals, such as mercury, or other toxic emissions,especially those considered to be persistent bioaccumulative toxins.

There are several different techniques for the reduction of each of theabove pollutants and coal-burning operations often utilize a combinationof some or all of these pollution control techniques in order to meetthe targeted emission levels. One example of these pollution controltechniques, namely, flue gas conditioning, is used to enhanceprecipitator performance. This technique deliberately deposits foreignchemicals, particularly ammonia, sulfur, and other proprietarychemicals, on the coal fly ash. This technique actually “conditions” thefly ash by coating the particles with these chemicals, changing thesurface conductivity and, therefore, the resistivity of the fly ash.These chemicals may also create a space-charge effect and improve thecohesiveness of the fly ash particles.

Injecting these chemicals in the hot flue gases will improve theefficiency of electrostatic precipitators and, therefore, the collectionrates for the coal fly ash. However, the collected fly ash will haveincreased levels of ammonia, sulfur oxides, and/or other residualchemicals which are known to negatively impact the marketability of flyash as pozzolan at high concentration levels.

Additional pollution control techniques include, but are not limited to,fuel switching and/or blending, the use of low-NOx burners, flue gastreatment to enhance the performance of NOx scrubbers, e.g., selectivecatalytic reduction (SCR), non-selective catalytic reduction, selectiveauto-catalytic reduction, etc., as well as the use of flue gasdesulfurization (FGD) scrubbers, etc. All these techniques have specificeffects on the fly ash which negatively impacts the marketability of flyash as pozzolan.

For example, there are several dry FGD scrubbing techniques that areused in coal burning operations to decrease the emissions of sulfuroxides, such as lime spray drying, duct sorbent injection, furnacesorbent injection, and circulating fluidized bed combustion. The use ofany of these techniques can result in a single, commingled by-productstream consisting of coal fly ash and spent lime sorbent. The generalmake-up of the residual particulate matter collected following thesecoal burning and dry FGD scrubbing operations, often genericallyreferred to as “spray dryer material,” are a heterogeneous combinationof coal fly ash and a blend of calcium sulfate and calcium sulfitecompounds.

The chemical composition of spray dryer material residues depends on thesorbent used for desulfurization and the proportion of fly ash collectedwith the FGD residues. The fly ash in dry FGD materials has similarparticle size, particle density, and morphology to those of conventionalfly ashes, but FGD materials have lower bulk densities. The differencein bulk density is due to variations in the chemical and mineralogicalcharacteristics of the reacted and unreacted sorbent. Dry FGD materialscontain higher concentrations of calcium and sulfur and lowerconcentrations of silicon, aluminum, and iron than fly ash.

Typically, dry FGD materials usually will not conform to the controllingspecifications for pozzolans (e.g., ASTM C-618), due to the varyingchemistry and glass content, the presence of high levels of calciumsulfate, and the generally heterogeneous nature of dry FGD materials.Therefore, they cannot be reliably used as pozzolan, especially forpozzolan in concrete structures.

In addition to the altered by-product particulate matter generatedthrough the use of these various clean air strategies, air emissionsfrom some of these pollution control techniques have in and ofthemselves resulted in other air pollutants. For example, at many powerplants, when flue gas undergoes selective catalytic reduction of NOx,high levels of SO₃ are emitted from the stack. The SO₃ is visible as a“blue plume” and quickly condenses into a mist of sulfuric acid,damaging the health of humans, animals, and plant life and destroyingreal property.

Therefore, coal-burning operations employing combinations of certain airpollution control techniques are now being forced to mitigate theunintended consequences of their actions by further altering theiroperations with additional flue gas treatments to limit emissions ofblue plume (SO₃) aerosols or other condensable particulate matter yet tobe determined and/or publicly reported in the literature.

In summary, coal burning operations have changed and will continue tochange in order to comply with federally mandated and/or self-imposedlimits on air emissions. These changes in coal-burning operationsinclude, but are not limited to, the use of low-NOx burners; fuelblending/switching, flue gas conditioning with ammonia or sulfur toenhance precipitator performance; flue gas treatment to enhance theperformance of NOx scrubbers; and/or FGD scrubbers to reduce theemissions of particulate matter or polluting gases, such as sulfur andnitrogen oxides.

Examples of residual chemicals and foreign particulate matter that maybe deposited in or adsorbed on coal fly ash include, but are not limitedto: (1) ammonia and/or SO_(3(solid)) from flue gas conditioning; (2)ammonia from NOx reduction scrubbing and/or slip; (3) the chemicalresiduals from injecting hydrated lime, magnesium hydroxide, sodiumbicarbonate carbonate, ammonia, sulfur, sodium bisulfate, magnacite,magnesium silicate, magnesium oxide, etc. for mitigating blue plume,i.e., SO_(3(gas)); and, (4) mercury-laden sorbents such as activatedcarbon from mercury scrubbing.

The presence of any of these foreign residual chemicals and/orparticulate matter will negatively impact the utilization of fly ash ingeneral and will especially negatively affect the value of fly ashmarketed as pozzolan in concrete.

The deterioration of fly ash quality referenced above negatively impactsthe value, marketability and, therefore, the utilization of fly ash inthe U.S. Specifically, a reduction in the pozzolanic reactivity reducesthe strength-producing characteristics; excessive unburned carbon isassociated with poor air-entraining characteristics; and chemicalresidues in the fly ash can negatively impact the marketability of flyash as pozzolan in these and other ways, creating additional technicaland aesthetic concerns.

It would be desirable to economically increase the value and utility offly ash in the marketplace by improving those characteristics of flyashes that have been identified by the concrete industry as beingdeleterious to the production of quality concrete; specifically, itwould be desirable to improve pozzolanic reactivity or strengthproducing characteristics, air-entraining characteristics, andcontamination from chemicals used for flue gas treatment.

BACKGROUND OF THE ART

A feature of the present invention is to provide a method and apparatusto thermally treat and, thereby, beneficially alter certaincharacteristics of low-Btu value fine particulate matter, especiallycoal fly ash, increasing the value of the processed fine particulatematerial, especially as pozzolan, over the value of by-product fly ashwhich has not been processed or otherwise beneficiated. This process isdesigned to expose fly ash to high temperatures in order to effectcertain physical and/or chemical changes which will: increase thepozzolanic reactivity and/or the amount of reactive glass surface area,improve the air-entraining characteristics by decreasing the level ofactivated carbon, and reduce the presence of chemical residualsdeposited in and/or on the fly ash during flue gas treatment.

Increasing pozzolanic reactivity derives mainly from increasing theamount and/or surface area of reactive glass in the fly ash. At thepresent time, there is very little published literature on the reducedpozzolanic reactivity of fly ashes produced through low-NOx burning ofpulverized coal. However, due to the lower combustion temperaturesnecessary to reduce the formation of thermal NOx, lower amounts of themineral matter contained in the coal are being converted to amorphousglass than before the deployment of low-NOx combustion techniques. Flyash processed through the present invention shows an increase in thespecific surface area of the glassy mineral matter, which correspondswith increased pozzolanic reactivity after processing with the presentinvention.

Specifically, the fly ash processed through the present invention showsan increase in the specific surface area of the glassy mineral matter,creating additional glass and increasing the fineness, through exposureto the unique circumstances caused by the present invention.

The processing temperature in the present invention is above the fusiontemperature of the mineral matter, causing the crystalline structure ofthe mineral matter to break down and become molten.

The particulate matter is reduced in size as carbon and othercombustible matter contained in the char particles oxidizes and/orvolatizes, significantly increasing the surface area of the particulatematter.

The coarsest particulate matter undergoes a disproportionate sizereduction, because the coarsest particles are deliberately subjected totangentially oriented, high velocity gas streams designed to createcentrifugal forces that increase particulate matter residence time inthe reaction zone and segregate coarser and/or denser particulatematter, which require the longest retention time to effect significantchemical and/or physical change, so that the coarser particles arecloser to the refractory-lined walls and the coarsest particles aresubjected to solid-to-solid contact with the refractory-lined walls. Thethermal mass provides an elevated heat source to expedite thermalreactions to combust the coarsest carbon char particles, which typicallycontain many smaller inorganic inclusions, thereby freeing up thatmineral matter to be finely divided and to become molten.

Additional heat energy is imparted directly to any remaining inorganicmineral inclusions, thereby converting additional mineral matter to amolten state, through the exothermic heat release of additional carboncombustion accomplished by exposure to downwardly oriented high velocitygas streams designed to create shearing forces that create turbulence,increasing kinetic rate of molecular transport for carbon redox. Thefinely divided molten particulate mineral matter remains in a finelydivided state, being separated by hot flue gases and being suspended andtransported by said hot gases while in a finely divided molten stateuntil laminar pneumatic transport patterns are re-established, therebyinhibiting agglomeration and, especially, fused or sintered productmaterial.

The molten mineral matter is quenched to below ash fusion temperaturethrough the evaporative cooling effect of spray water injected into thehot gas stream in order to solidify the mineral matter as glass,completing the vitrifying process with a tempering effect which isdesigned to increase the glass-to-crystalline ratio and the reactivityof the amorphous mineral matter to a greater extent than would normallybe accomplished through the typical annealing effect of coal-burning orexisting fly ash carbon combustion processes, which are more slowlycooled by the radiant heat transfer necessary for maximum heat recovery.

Other processes are known that utilize fly ash to manufacture productswith high glass content. One method describes a formulation that usesfine coal ash as a constituent for soda-lime glass, wherein the fly ashbecomes a part of the soda-lime glass melt. In one method the fly ash isadded and mixed with other constituents to form the molten glass. At theelevated temperatures required for soda-lime vitrification, anycrystalline mineral matter in the fly ash would become molten and becomea part of the melt. However, the fly ash becomes a part of a new glassproduct and does not remain in the finely divided state required to beused as pozzolan.

Another reported method uses fly ash from incinerated solid wastematerials to produce an inert vitrified ash product and coal fly ash toproduce ceramic products and glazing, respectively. In these and othersimilar methods, fly ashes or other fine particulate matter is used toproduce vitrified products. It would be desirable to have a method toprocess finely divided fly ash or other particulate matter so as to bothincrease the glass to crystalline ratio of the material and to remain asa finely divided material suitable for use as pozzolan.

Many processes have been described that thermally treat fly ash andwhich also attempt to keep the fly ash as a finely divided materialsuitable for use as pozzolan. However, these methods are intended tocombust residual carbon in fly ash or volatize ammonia on fly ash and donot operate at the high temperatures necessary to melt mineral matter.One method claims lower operating temperatures to stay below ash fusiontemperatures and, thereby avoid sintering fly ash in fluid bed carbonburn out operations.

The present invention can operate at the high temperatures required tomelt the crystalline structure of the mineral matter and yet avoidssintering or other thermal agglomeration because the process maintainshigh temperatures in the reactor while the hot flue gases re-establishnon-turbulent, laminar flow patterns which ensure that individual moltenparticles remain separated while suspended and transported in the hotflue gases until after the quenching spray water reduces the temperaturebelow the ash fusion temperatures.

Other prior art methods teach a thermal treatment in a fluid bed of amixture of particulate matter which includes fly ash and other alkalicompounds. However, this thermal processing is specifically intended andcontrolled to reduce the glass content and increase the crystallinecontent of the resultant product. Conversely, the process of the presentinvention is specifically intended and controlled to increase theglass-to-crystalline ratio of the fine particulate matter and thepresent invention does not require processing in a fluid bed.

The thermal treatment processes a mixture of particulate matter whichincludes fly ash and other alkali compounds in a fluid bed. However, thethermal processing is specifically intended and controlled to reduce theglass content and increase the crystalline content of the resultantproduct. It would be desirable for a treatment to increase the glass tocrystalline ratio of the fine particulate matter and the presentinvention does not accomplish processing in a fluid bed. It would bedesirable to have a method for particle size reduction without the useof fine grinding or other mechanical devices.

A secondary, related feature of the present invention is to increase thefineness of particulate matter, especially fly ash, by decreasing thesize of the carbon char particles. One existing method is a treatment offly ash that includes fine grinding of fly ash with at least one othercompound, thus assuring that the resultant product is finely divided andpresumably suitable for use as pozzolan.

There are other processes which have been described that effect particlesize reduction through fine grinding or other mechanical means. It wouldbe desirable to have a method for particle size reduction without theuse of fine grinding or other mechanical devices.

Typically, the coarsest particles in a fly ash are unburned coal char orcoke. Being similar to coal, although essentially devolatized, thesechar particles contain smaller inclusions of inorganic mineral matter.It would be desirable to have a method which can be operated in such away as to combust the residual carbon char in the fly ash. Individualburning char particles have been described as a shrinking carbon core;as the carbon surrounding the mineral matter volatizes, the size of theindividual char particles is reduced significantly. Also, a desirablemethod should specifically process the coarsest fly ash char particlesin a deliberate fashion to disproportionately expedite a shrinkingcarbon core for these particles, thereby freeing the smaller inorganicinclusions from the larger char particles, creating new, very fineparticulate matter.

Another feature of the present invention, improving the air-entrainingcharacteristics of the fly ash, derives through thermal oxidation toeffect a combination of reduced carbon surface area and destructionand/or occlusion of active carbon sites.

The AEA adsorption efficacy of residual unburned carbon in fly ash isaffected by the amount, type, and availability of activated sites. Thepresent invention will treat fly ash through thermal oxidation and,thereby, may, depending on operating conditions: 1) reduce the amount ofcoal coke (i.e., lower LOI) through combustion, and/or 2) oxygenateactive carbon sites (i.e., increase LOI) through oxygen deposition,and/or 3) occlude active carbon sites by decreasing the critical ashporosity and/or increasing the ash film thickness surrounding coal charparticles.

It is not necessarily a goal of this invention to accomplish any ofthese treatments individually; rather, the fly ash would be treated tomaximize the beneficial effect on the air-entraining characteristics ofthe fly ash. However, depending on the operating parameters of thepresent invention and the characteristics of the fly ash beingprocessed, any of the above listed treatments could be individuallyaccomplished. Some of the above listed treatments have been attemptedand, in some cases accomplished, either commercially or in thelaboratory. However, there is no prior art teaching of one method orapparatus to accomplish the combination of all these effects in onetreatment. Also, the apparatus of the present invention is unique andoperates using a fast fluidized regime, therefore, none of the specific,individual treatments listed above have ever been accomplished in anapparatus and/or operating regime taught by the present invention.

An apparatus or method which will treat fly ash and thereby modify theash film boundary of coal char particles to increase the ash filmthickness and/or density (and reduce the porosity), thereby increasingthe coverage of the ash film boundary, occluding active carbon sitesand/or impeding the adsorption of AEA has not been taught.

A number of technologies have been explored to try to effect carboncombustion in fly ash to reduce the carbon levels as low as possible.The primary problems that have faced most commercial methods in recentyears generally have been the operational complexity of such systems andmaintenance issues that have increased the processing costs per ton offly ash processed, in some cases, to a point where it is noteconomically feasible to use such methods.

Another method describes a process in which the ash is conveyed inbasket conveyors and/or on mesh belts through a carbon bum out systemthat includes a series of combustion chambers. A further methoddescribes a process in which the finest carbon fraction of the residualcarbon in fly ash is burned. Others have described processes wherebyknown ash feed or conveying systems transport ash using conveyors, screwmechanisms, rotary drums and other mechanical transport devices throughone or more combustion chambers.

At the high temperatures typically required for ash processing, however,such mechanisms have often proved difficult to maintain and operatereliably. In addition, such mechanisms typically limit the exposure ofthe carbon particles to free oxygen by constraining or retaining the ashwithin baskets or on mesh belts such that combustion is occasioned by,in effect, diffusion through the ash, thereby retarding the effectivethroughput through the system. Consequently, fly ash carbon residencetimes within the furnace also must be on the order of upwards of 30minutes to affect a good burn out of carbon. All of these factorsgenerally resulting in a less effective and costlier process and havenever been commercialized.

Crafton et al. (U.S. Pat. No. 6,521,037) describes a fluid bedcombustion process designed to reduce the LOI to below 2%. Also, U.S.Pat. No. 5,160,539 and U.S. Pat. No. 5,399,194 of Cochran describeprolonged roasting in a fluid bed to reduce the LOI in a temperaturerange from 1300 to 1800° F. (700 to 982° C.) and 800 and 1300° F. (426and 700° C.), respectively.

In early work, by Cochran, two types of “transport reactors” were testedwith residence times on the order of 1 to 15 seconds. In these transportreactors all of the reactants travel together at more or less the samespeed. In this early work, the large volume of air necessary to providesufficient stoichiometric oxygen for carbon burnout was used totransport the reacting fly ash from inlet to discharge points.

However, no substantial carbon burnout was detected and Cochrandiscontinued development of a pneumatic transport solid gas reactor.Ultimately, in order to increase retention time at high temperature,Cochran developed the bubbling fluid bed reactor (U.S. Pat. Nos.5,160,539 and 5,399,194). The present invention teaches an apparatus andmethod for thermal processing, including carbon combustion, using apneumatic transport solid gas reactor.

In all the prior art cited above for thermal processing of fly ash, thespecific design and clear intent of these inventions is to eliminate asmuch carbon as possible. This requires that the fly ash particles besupplied with sufficient temperature, oxygen and residence time, usuallya prolonged period of time roasting in a heated chamber, to cause thecarbon within the fly ash particles to ignite and burn, leaving cleanash particles.

U.S. Pat. No. 6,783,585 of Zacarias, et al. teaches a thermal methodwhich preferentially combusts only the finest fraction of the residualunburned carbon in fly ash but this process does not effectively treatactive carbon sites or to combust or otherwise treat the coarse carbonparticles. The present invention treats all the residual carbon in thefly ash and specifically processes the coarsest fly ash char particlesin a deliberate fashion to accelerate a shrinking carbon core, therebyexpediting the development of a thicker ash film boundary to encapsulateor occlude any remaining active carbon sites.

Another approach to beneficiating fly ash relates to chemically treatingthe residual unburned carbon in fly ash. These chemical treatmentsemploy a range processes and oxidizing reagents from liquid peroxidesand nitric acid to ozone gas. However, all generally intend to oxidizethe residual unburned carbon in an attempt to de-activate the“activated” carbon in the fly ash, usually by depositing an oxygenmolecule on the exposed surface of active sites on the residual carbonin fly ash, thereby satiating the propensity of the carbon to adsorb theAEA dosed in the concrete. This de-activation of the activated carbon issometimes called “passivation.” All these approaches to beneficiatingfly ash occur at ambient temperatures.

The present invention does effect the same chemical and physical changes(i.e., oxidation and passivation) as these other treatments; however,the present invention employs an entirely different method, inparticular, a high-temperature operating regime and uses ambient oxygenin atmospheric air as the reagent.

Another feature of the present invention, reducing the presence offoreign chemical residuals deposited on the fly ash during flue gastreatment, derives mainly from the chemical and/or physical changes,including changes in chemical speciation, disassociation, and/ordecomposition, to these chemical residuals in the high temperature andoxygen-rich operating regime of the present invention.

Furthermore, the apparatus includes the infrastructure to direct othermaterials, including gases, liquids, solids, or combinations thereof,which would act as additional reagents to facilitate the desiredchemical and/or physical changes and the temperature in the reactor canbe optimally maintained to expedite said reactions as desired.

For example, ammonia is used both for flue gas conditioning and flue gastreatment and, consequently, significant levels of ammonia may bedeposited on the fly ash, reducing the volume, the value, and(potentially) the safety of fly ash utilized in concrete. Any ammoniadeposited on fly ash processed with the present invention will undergochemical decomposition, converting ammonia to nitrogen and water vapor.

Specific operating temperatures, air flows, and retention times forbeneficiating fly ash and/or other particulate matter with the presentinvention will vary according to the processing constraints required toeffect the desired physical and/or chemical changes. These processconstraints, such as the temperature/time requirements to effectcombustion, oxidation, oxygenation, ash fusion,volatilizatioi/condensation, changes in speciation, etc., for theparticulate matter itself, as well as the various contaminants targetedfor processing, are individually known by those skilled in the art and,do not need to be disclosed in detail to describe the present invention.

For example, the chemical composition of spray dryer material residuesdepends on the sorbent used for desulfurization and the proportion offly ash collected with the FGD residues. The fly ash in dry FGDmaterials has similar particle size, particle density, and morphology tothose of conventional fly ashes, but FGD materials have lower bulkdensities. The difference in bulk density is due to variations in thechemical and mineralogical characteristics of the reacted and unreactedsorbent. Dry FGD materials contain higher concentrations of calcium andsulfur and lower concentrations of silicon, aluminum, and iron than flyash.

Processing such spray dryer material residues through the presentinvention will have the effect of burning, and thereby volatizing, someof the combustibles present in the particulate matter. Therefore, thespecific particulate matter which combusts at certain operatingtemperatures will have the effect of changing the overall chemistry ofthe resultant processed material, since a portion of the particulatematter that is combustible will volatize as flue gas, leaving as solidresidue primarily the non-combustible mineral matter.

Also, the ash fusion temperature of such spray dryer material residueswill be significantly lower than the ash fusion temperature of the coalthat is burned due to the significant increase in alkalinity from theresidual particulate matter deposited in the fly ash from thecalcium-based sorbents. Consequently, processing such material throughthe present invention can dramatically increase the glass to crystallineratio, increasing the pozzolanic reactivity such that the resultantmaterial would meet the strength activity requirements of thecontrolling specifications.

However, the specific operating conditions required to effect thenecessary chemical and physical changes needed to manufacture apozzolan-grade material from dry FGD material will vary from site tosite and disclosing those operating conditions would compromise obviouscompetitive advantages endemic to that knowledge.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the drawings in which like referencecharacters designate the same or similar parts throughout the figures ofwhich:

FIG. 1 is a schematic view of one exemplary embodiment of an apparatusof the present invention.

FIG. 2 is a cutaway perspective view of one exemplary embodiment of areactor according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention generally relates to the beneficial thermalprocessing of low-Btu value fine particulate matter, such as, but notlimited to, fly ash, in order to remove specific targeted contaminantsdeposited in or on said particulate matter or otherwise augment theparticulate matter through concentration or through reaction withadditional reagents. For the purposes of the present disclosure fly ashwill be discussed as a nonlimiting example of particulate matter whichcan be processed using the methods of the present invention. Other fineparticulate matter, such as, but not limited to, spray dryer material(also known as FGD materials) or the like, may be used, possibly withmodification of the apparatus or method parameters in ways known tothose skilled in the art. More particularly, the present inventionrelates to a method, apparatus and the products derived from processingfly ash in a reactor designed to expose fly ash to the intimate presenceof a sufficient gas reactant (especially oxygen), at the requiredtemperature, and for the required time to effect certain physical and/orchemical changes that increase the value of the fly ash when used aspozzolan in concrete. Specifically, these changed characteristics willincrease the pozzolanic reactivity and/or the amount of reactive glasssurface area, improve the air-entraining characteristics by decreasingthe level of activated carbon, and reduce the presence of chemicalresiduals deposited on the fly ash during flue gas conditioning and/ortreatment.

Another feature of the present invention is to manufacture higher-valuepozzolans from and/or co-process fly ash with dissimilar raw feedmaterials (e.g., other than coal fly ash), which have little or no Btuvalue and which can be processed at high temperatures to manufactureother products possessing pozzolanic properties or other knownpozzolans, such as calcined clay, metakaolin, rice hull ash, etc. Forexample, baghouse fines from aggregate crushing operations may bechemically similar to fly ash and have a similar particle-sizedistribution, but, because the particulate matter is almost exclusivelycrystalline, the pozzolanic reactivity is almost non-existent. Thesebaghouse fines could be processed using the present invention separatelyor co-processed with coal fly ash. When processed through the presentinvention, the baghouse fines would be transformed into a finely dividedglass with significant pozzolanic reactivity.

In one exemplary embodiment of the present invention, shown in FIGS. 1and 2, an apparatus 5 comprises a refractory-lined reactor vessel 10,wherein the refractory is heated by burners 11 to a sufficiently hightemperature to provide the required thermal mass to elicit the desiredchemical and/or physical changes in the fly ash or other fineparticulate matter. The burners 11 are used for start-up; however,continuous isothermal processing can be maintained through the input ofheat released by the exothermic reaction of burning residual carbon inthe fly ash. Of course, if there is not enough fuel value in the fly ashfor self-sustaining combustion and/or to maintain a heat balance at thedesired target temperature, the burners 11 would be used to maintain therequired temperature.

Raw feed fly ash, which may be preheated if desired, is conveyed intothe reactor 10 through the fine particulate feed ports 12. Ambient airor other gas, which may also be preheated if desired, serves as theprimary reagent and is conveyed at high velocity into the reactor 10through a number of swirl-inducing nozzles 13 or manifolds with severalheads which can arranged in a single row, multiple rows, staggered or inother configurations, and designed to induce both swirling and turbulentair flow patterns, which help facilitate mixing and generally expeditingthe desired thermal reactions.

For the purposes of the present disclosure, “turbulence” or “turbulent”is defined as a state of being highly agitated and turbulent flow isfluid flow in which the velocity and/or direction of a given particlegenerally continually changes. Flow that is not turbulent is calledlaminar flow.

Particularly, the overall volume of air conveyed into the reactor 10 issufficient in ambient oxygen or other reagents to provide the necessarystoichiometric ratio of reactants for the targeted level and types ofreactions, such as combustion, oxidation, oxygenation, speciationchange, etc., requiring only the supply of sufficient temperature and/orretention time with residual impurities integral to, deposited on, orcommingled with the raw feed fly ash. The overall volume of air is alsosufficient to provide the necessary overall average velocity through thereactor to vertically transport substantially all the fly ash throughand out of the reactor 10, through a high-temperature cycloniccollector/separator 18, through an optional air pre-heater 19, and intoa baghouse, or other similar device, for collection.

A portion the air may be conveyed into the reactor 10 through the swirlair nozzles 13 at high velocity, creating a cyclonic swirling reactionzone by which centrifugal forces created by the swirling effect movecoarser and/or denser particulate matter to the reaction chamber 10walls 10A, segregating the particulate matter and providing asolid-to-solid contact zone with the refractory-lined walls 10A in theoutside region of the reactor 10, thereby increasing the residence timeof the particulate matter in the reaction zone and imparting the longestresidence times for the coarsest particles as these particles travel thelength of the inside circumference of the reactor 10 many times in agenerally upward, helical or spiraling path before exiting the swirlingreaction zone of the reactor 10. The angle of the nozzles 14 can be setdepending on the particular effect desired (such as, but not limited to,carbon reduction, carbon contact with the refractory walls, or thelike).

Also, at least a portion of the air may be conveyed into the reactor 10through the turbulent air nozzles 14 or manifolds with several headswhich can arranged in a single row, multiple rows, staggered or in otherconfigurations, at high velocity, creating a turbulent reaction zone, bywhich the downward shearing action of high velocity air streams furtherincreases particulate matter residence time in the reaction zone andimparts sufficient kinetic force to disrupt upward and/or laminarswirling air patterns, thereby promoting sufficient mixing of solid andgaseous reactants and/or turbulent scrubbing, which facilitates the masstransfer of reagent molecules, especially ambient oxygen in the highvelocity air stream, to effect the desired chemical reaction(s). In oneexemplary embodiment the nozzles 14 are arranged in a generallyhorizontal linear array. The nozzles 14 are preferably angled downward.

The overall air volume may be further divided into a portion for underfire air conveyed into the reactor 10 through a number of “under fire”air ports 15, inhibiting particulate matter fallout, and other portionsmay be conveyed into the reactor 10 through a number of “over fire” airports 16, providing sufficient staging and/or total stoichiometricreagent air for the desired effect(s).

As the individual particles of a fine particulate matter, such as flyash, are exposed to the high temperature within the reactor 10, theparticles are heated to elicit the desired chemical and/or physicalchanges. The processing temperature is preferably above the ignitiontemperature of any residual unburned carbon; therefore, there will besome reduction in the carbon content of fly ash through combustion. Thetotal amount of carbon reduction will depend on the retention time inthe high temperature regime and that retention time will be adjusted, asneeded, by altering the velocity of the air flow and/or the amount ofrecirculation of fly ash, by use of the high-temperature cycloniccollector/separator 18 (or other suitable particle size separationdevice), and/or introduction of or other fine particulate matter.

In some cases, the processing temperature will be above the fusiontemperature of the fly ash mineral matter and/or the fusion temperatureof chemical and mineral residuals present in the raw feed fly ash inorder to increase the glass-to-crystalline ratio of the mineral matterand/or to reduce the presence of chemical residuals deposited on the flyash during flue gas treatment at the coal-burning facility. In thesecases, the process of the present invention in one exemplary embodimentincludes a method, preferably through the use of a number of spray waterinjection nozzles 17 preferably located in the reactor 10 along the topportion, to quickly quench the flue gases and any molten mineral matterwhile suspended in the flue gas stream in order to increase the quantityand reactivity of the glass and assure that the particulate matterremain in a finely divided state. Preferably, a second set of spraywater injection nozzles 17A are located in the conduit 17B between thereactor 10 and the separator 18.

In other cases, the processing temperature will be maintained at orabove the required temperatures to effect volatilization, decomposition,and/or the desired change in chemical speciation of chemical residualsdeposited on fly ash during the original coal-burning and/or flue gastreatment. Examples of such chemical residuals include, but are notlimited to, the residue from hydrated lime, magnesium hydroxide, sodiumbicarbonate carbonate, ammonia, sulfur, sodium bisulfate, magnacite,magnesium silicate, magnesium oxide, and residue from other chemicalsco-fired and/or co-processed at coal-burning facilities.

In some cases it may be advantageous for the processed particulatematter to be separated from the flue gas stream through the use of thehigh-temperature cyclonic collector/separator 18 or other suitableseparation device at or above certain specified temperatures to allowefficient recirculation of heated particulate matter through the reactorto increase exposure and/or retention time in the reaction zone untilthe desired physical and/or chemical changes are fully effected; and thehigh-temperature separation of the processed particulate matter fromtargeted contaminants that have been volatized and dispersed into theflue gas while still above the condensation temperature of thecontaminant(s) so as to reduce the level of that contaminant in theproduct fly ash.

In other cases it may be advantageous to co-process other substanceswith coal fly ash processed with the present invention by conveyingforeign particulate matter, gases, or liquids into the reaction zonethrough the fine particulate feed ports 12, swirl and turbulent airnozzles 13 and 14 or under-fire/over-fire air ports 15, 16, or, spraywater injection nozzles 17 and/or 17A, respectively, which may serve toreduce the liability of and/or add value to the beneficiated product inthe commercial marketplace. The unique operating regime of the presentinvention, especially the high temperature and the high velocity airmixing, provides an excellent environment to facilitate chemical andphysical processing for many types of particulate matter, including bothfly ash and non-ash raw feed materials, as well as expediting thereaction times for many gases and liquids used as reagents to inducecertain beneficial effects.

In all cases the fly ash will finally be cooled by being conveyed to andpassing through one or more heat exchangers 20 or other cooling deviceto the lower temperatures expected for use as pozzolan. Depending on theintended effect, the apparatus may be operated so that the fly ash iscooled separately from the flue gases or along with the flue gases asthe fly ash and flue gases pass through the heat exchanger 20, a wasteheat recovery boiler, and/or other functionally similar devices. Aftercooling, the material passes to a gas-solids separator, such as abaghouse 22, there solids are collected and gases are removed. Solidsare bagged or otherwise packaged for storage or transport.

The present invention can effect LOI reduction in seconds rather thanminutes; and efficient operation of the present invention will attaintargeted LOI reduction without the prolonged residence time in aroasting chamber; and thereby, product throughput per unit of time isincreased; therefore, the present invention does claim carbon reductionthrough combustion by use of the unique apparatus and process describedherein. Consequently, a secondary object of the present invention is tolower the LOI enough to conform to the prescribed maximum LOI limits ofcontrolling specifications for pozzolan-grade fly ash.

Although only a few exemplary embodiments of this invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention as defined inthe following claims. It should further be noted that the disclosures ofany patents, applications and publications referred to herein areincorporated by reference in their entirety with regard to their partsrelevant to the present invention.

1. A method for processing low-Btu value particulate matter, comprising:a) introducing into a reactor chamber a feed of particulate mattercomprised of combustible and incombustible matter; b) heating saidparticulate matter; c) introducing ambient air into said combustionchamber at a high velocity through a plurality of nozzles, said nozzlesarranged to induce laminar swirling and/or turbulent air flow patternsin said reactor chamber; and, d) separating processed particulate matterfrom said air flow.
 2. An apparatus for beneficiation of fly ash orother material, comprising: a) a refractory-lined vessel; b) at leastone heat source for heating; c) a plurality of swirl-inducing nozzles;d) a plurality of turbulence-inducing nozzles for creating turbulent airflow within said vessel; e) means for introducing said fly ash into saidvessel; f) a heat exchanger for cooling said fly ash material; and, g)means for removing separated material from said vessel.